This is the world's simplest problem to solve
The angle of reflectance always equals the angle of incidence, so all that you have to do is to make sure that your lighting is at an angle that doesn't reflect into the lens.
At its simplest, what you need is a softbox that is at least equal in size (can be bigger) than the shortest edge of the painting, place it over to one side at an angle of 45 degrees.
But this will cause another problem because the side nearest to the softbox will receive far more light than the side furthest away (inverse square law) which means that you need an identical softbox on the other side too.
Make sure that the camera is dead square to the painting.
This lighting will avoid all unwanted reflections but isn't always ideal for some designs of frame.
A polariser isn't needed but won't do any harm and occasionally may help slightly.